Heat-hardenable reaction resin mixtures are disclosed by British Pat. Nos. 1,585,339, 1,474,371; and 1,421,460, and German Pat. Nos. DE-OS 27 22 400, DE-OS 24 32 952 and DE-OS 23 08 802. These known mixtures of polyisocyanates/epoxide resins and unsaturated olefinic compounds, produce molding materials having high thermal dimensional stability, long-term heat resistance and good mechanical and electrical properties when cross-linked in the presence of certain accelerator systems. The use of these known mixtures has not been entirely successful due to technical difficulties associated with the accelerators or accelerator systems described in the above-named publications which cause the mixtures to harden either too slow or too fast. Thus, it has not been possible, for example, to economically produce insulations with known mixtures of polyisocyanate, epoxy resin and olefinically unsaturated compounds and accelerator systems because the viscosity of these mixtures either increases too fast interfering with complete impregnation or, too slow, so that hardening in an economically feasible time does not take place.